It shouldn't need any kind of shim. The back of the I-61 rotates as you focus and that's possibly fouling against the body. There isn't really any way to shim that out and if you did, you'd throw the lens forward and upset the focus anyway. Try not screwing it on quite so tightly, for starters (I don't mean leave it loose, of course). The lens may also benefit from a re-lube, despite seeming smooth when off the body.
You might also take the back off and hold the shutter open on "B" and then have a look to see if you can see anything catching as the focus is turned. One other thing to check: is the lens stiff on its mounting threads? It could be distorted and putting strain on the mount. The mount is pretty soft aluminium and not difficult to distort. I've seen that issue on Zorki/FED 1 models.
